Limit switch circuit with vacuum tube servo-amplifiers



April 28, 1959 F. H. KALTENBACH 2,384,579

LIMIT SWITCH cmcun WITH VACUUM TUBE SERVO-AMPLIFIERS Filed Feb. 15, 1956 IN VEN TOR.

Frederick H. Kalrenbach BY ATTORNEY arT I United States Patent() LIMIT SWITCH CIRCUIT WITH VACUUM TUBE I SERVO-AMPLIFIERS hederick H. Kaltenbach, Alliance, Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Application February 15, 1956, Serial No. 565,598

9 Claims. (Cl. 318-207) This invention relates to servomotor systems of the type including limit switches, and, more particularly, to a novel servomotor system of this type including novel means for maintaining stable sensitivity of the system when a limit switch is operated.

A well-known and widely used servomotor system employs a pair of grid controlled electronic valves connected in phase detecting relation to operate a reversible electric motor in selected opposed directions in accordance with the relative phase of a suitably amplified signal input voltage applied to the control grids of the valves. The motor may be, for example, a split phase condenser type motor operating a valve or the like. At either limit of movement of the operated member, a limit switch is operated to prevent further movement in the same direction while permitting movement in the reverse direction when the input signal calls for such reverse movement.

. As the limit switch included in the energizing circuit for such reverse direction rotation of the motor remains unoperated when the forward limit switch has thus been opened, there is a tendency for the motor to creep back if the control system is maintained at a desired sensitivity to the input signal. Attempts to eliminate such creep back tendency have resulted in undesirable loss of sensitivity in the system.

In accordance with the present invention, an improved servomotor circuit of the limit switch type is provided in which the tendency of the motor to creep back when a limit switch is operated is eliminated without in any way reducing the sensitivity of the system to an input signal calling for operation in the reverse direction.

More specifically, a pair of relays are provided each operated whenever the associated limit switch is operated at the limit of operation in one direction. Each relay includes a normally closed switch and a pair of normally open switches. When a relay is operated by operation of its controlling limit switch, the normally closed relay switch is opened to interrupt the signal input to the control grid of the then dominant electronic valve. At the same time, the two normally open relay switches are closed. One of these latter switches shunts the control grid of the dominant electronic valve to ground. The other normally open relay switch connects a diode between the control grid of the other electronic valve and ground. As a result, the two electronic valves are returned to the stable condition existing when there is no signal input, and the motor is held against movement. However, if a reverse input signal is received, such other electronic valve causes the motor to turn in the reverse direction, restoring the previously operated limit switch and returning the system to normal.

For an understanding of the invention principle, referonce is made to the following description of a typical embodiment thereof as illustrated in the accompanying drawing.

In the drawing the single figure is a schematic wiring diagram of a servomotor limit switch system embodying the invention.

2,884,579 Patented Apr. 28, 1959 Referring to the drawing, the invention is illustrated as embodied in a servomotor system for operating a splitphase motor M including an armature 10 and windings 11 and 12. Winding 11 is connected across a supply circuit comprising conductors 1313 extending from terminals 14--14 connected to a suitable supply of alternating current, winding 11 being connected across conductors 13-13 in series with a condenser 15. Conductors 13-13 also supply current to the primary winding 21 of a transformer 20 having secondary windings 22 and 23. Winding 22 supplies the low voltage filament heating supply for the system, whereas winding 23, through a rectifier 24 and by virtue of a by-pass condenser 26, supplies B+ potential for the system.

The second motor winding 12 is arranged to be energized, selectively as to relative phase, through the medium of a pair of grid controlled electronic valves V-1 and V2 which, in the illustrated example, are vacuum tubes and may be a pair of independent tubes or may be two triodes included in the same envelope. For this purpose, a transformer secondary Winding 25 is connected at its opposite ends to the plates or anodes 2727 of tubes V-1 and V-2, the primary winding (not shown) coupled to winding 25 being connected to the AC. supply circuit. The cathodes 2S-28 of the two tubes are commonly connected to ground through a resistor 29. A conductor 31 connects the midpoint of winding 25 to one end of motor winding 12, the other end of this winding being grounded.

The control grids 30 of valves V-l and V-2 are arranged to have a signal potential applied thereto to control the rotation and direction of operation of motor M, which later may, for example, operate a valve or any other controlled member, the control signal being variable in phase in accordance with the desired direction of rotation of the motor. The control may be applied in any desired manner such as, for example, by applying the output of a telemeter coil directly to amplifier 45, or by applying a D.C. signal of reversible polarity through a chopper or inverter to transformer 40. By way or giving a specific example, and not by way of limitation in any manner, the control signal is illustrated as applied to signal input terminals 33-34. Terminal 33 is connected to a suitable signal sense selector 35, illustrated, solely by way of example, as a switch arm selectively engageable with either of a pair of contacts 3636 connected to the outer ends of series connected primary windings 41-41 of transformer 40. The midpoint or junction of primary windings 41 is connected to terminal 34. The signal voltage from transformer secondary winding 42 is amplified by amplifier 45 and the amplified signal voltage is taken from output terminal 46 of the amplifier. Output terminal 46 is connected by a conductor 47 through a condenser 48 to grid 30 of valve V-1. Similarly, output terminal 46 is connected by a conductor 51 through a condenser 52 to grid 36 of valve V-2. Each of the grids 30 is connected to ground through a biasing resistor 37.

In ordinary operation, motor M is stationary when no signal is applied to grids 30 of valves V-l and V-Z, as both valves conduct equally thus cancelling the tendency of each to cause the motor to rotate. Should an A.C. signal be received, due to, for example, movement of selector 35 into engagement with one or the other of contacts 36, it is fed into amplifier 45 with its relative phase dependent upon which of contacts 36 is engaged. Upon receipt of such ,a signal by amplifier 45, an unbalanced condition is created causing one valve to increase its current and the other valve to decrease its current. This provides a directive effect on the current output of the two valves, and the resulting current flow causes the motor to rotate in a selected direction. Should the signal-phase be shifted as by selector 35 en. gaging the other contact 36, the electronic valve action is reversed as to current-flowdirection, thus reversing the relative phase of the current supplied to the motor winding 12. This causes motor M to operate in the reverse direction.

At either limit of the operation of the control member, such as a valve for example, a limit switch 50 or 60 .is operated either directly by the motor or by the controlled member. For convenience, limit switch 50 will be referred to as the forward limit switch whereas limit switch 60 will be referred toas the reverse limit switch, these references being solely relative. Limit switch 50 is connected across conductors 13-13 in series with a forward relay F, and limit switch 60 is connected across conductor 1313 in-se'ries with a reverse relay R. Relay F includes a normally closed relay switch F-l and a pair of normally open relay switches -F-2 and .F-3'. Similarly, relay R includes a normally closed relay switch R-1 and a pair of normally open relay switches -R-2 and R-3. connected in series between condenser 48 and the control grid 30 valve V-l, whereas normally closed relay switch R4 is connected in series between condenser 52 and con- -trol grid 30 of valve V-2. Relay switches F-2 and R-2 -are respectively connected in shunt with the-biasing resistors 37 for the control grids of valves V-1 and V-2 respectively. Relay switch F-3 is arranged, when'closed, to connect a diode 55 between the control grid of valve V4 and ground. Similarly, relay switch R-3 is arranged, when closed by energization of relay R, to connect a diode 65 between control grid 30 of valve V-1 land ground. While the limitswitches 'are illustrated as normally open and the relays as normally dropped, the arrangement is equally efiective with normally closed limit switches operating normally energized or'picked up relays. The particular arrangement illustrated operates in the following manner. Shouldthe controlled member operfated by motor M reach its limit of movement in the for- ;ward direction, limit switch 50 is closed. During such forward movement, the relative phase of the input signal is such that the current of valve V-l is increased and that of valve V-2 is decreased. When limit switch 50 is thus closed, relay F is energized to open relay switch F-1 and close relay switches F-2 and F-3. Opening of relay switch F1 interrupts the control signal supply to control grid 30 of valve V-l, closure of relay switch F-2 connects this control grid to ground, and closure of relay switch F-3 connects diode 55 between control grid 30 of valve V-2 and ground. This results in the two valves returning to their stable condition with no eflective operating current being applied to motor M. Should the signal switch now be shifted 180, the amplified signal current applied to grid 30 of valve V-2 causes an increase in the current flow through this valve and thus a 180 reversal in phase of the current through motor winding 12. This causes the motor to operate in the reverse direction and, as the motor backs up, limit switch 50 is re-opcned. Opening of swiftch 50 drops relay F to re-close relay switch F-l and re-open switches F-2 and F-3, restoring the system to the normal condition illustrated in the drawing.

An analogous sequence of operations occurs when'the Normally closed relay switch F-l is operated member reaches its limit of movement in the connected, in phase-detecting relation, to said winding; signal input means operable to apply a signal voltage to the grid circuits of said valves; a pair of diodes each connected to the grid circuit of one of said valves; and switch means, connected between the grid of each valve and the signal input means and between each diode and ground, and operable by said motor at its limit of operation-ineach-direction to disconnect the grid circuit of one valve from said signal input means, and to connect one of said diodes across the grid circuit of the other valve; said switch means being operable, upon a reversal in the direction of operation of said motor, to reconnect the grid circuit of said one valve to said signal input means, and to disconnect said diode from the grid circuit of the other valve. p v

2. A servo-motor" circuit comprising, in combination, an A.C. motor having a winding controlling its direction of rotation; a pair of grid controlled electronic valves connected, in phase-detecting relation, to said winding; signal input-means operable to apply a signal voltage to the grid circuits of said valves; a pair of diodes each'connected to the grid circuit of one of said valves; and switch means, connectedbetween the grid of each valve and the signal input means and between each diode and ground, and operable by said motor at its limit of operation in each direction to disconnect the grid circuit of one valve from said signal input means and ground such disconnected grid circuit, and to connect one of said diodes across the grid circuit of the other valve; said switch means being operable; upon a reversal in the direction of operation of said motor, to reconnect the grid circuit ofsaid one valve to said signal input means and to remove such ground, and to disconnect said diode from the grid circuit of the other valve. g

3. A servo-motor circuit comprising, in combination, an A.C. motor having a winding controlling its direction of rotation; a pair of grid controlled electronic valves connected, in phase-detecting relation, to said winding; signal input means operable to applya signal voltage to the grid circuits of said valves; a pair of diodes each connected to the grid circuit of one of said valves; a pair of relay means each having armatures connected between the grid of one valve and the signal input means and between one diode and ground, and each operable to disconnect the grid circuit of one valve from said signal input means, and to connect one of said diodes to ground across the grid circuit of the other valve; and switch means operable by said motor at its limit of operation in each direction to selectively operate one of said relay means; said switch means being operable, upon a reversal in the direction of operation of said motor, to restore the operated relay means to reconnect the grid circuit of said one valve to said signal input means, and to disconnect said diode from the grid circuit of the other valve.

4. A servo-motor circuit comprising, in combination, an A.C. motor having a winding controlling its direction of rotation; a pair of grid controlled electronic valves connected, in phase-detecting relation to said winding; signal input means operable to apply a signal voltage to the grid circuits of said valves; a pair of diodes each connected to the grid circuit of one of said valves; a pair of relay means each having armatures connected between the grid of one valve and the signal input means and ground and between one diode and ground, and each operable to disconnect the grid circuit of one valve from said signal input means and ground such disconnected grid circuit, and to connect one of said diodes to ground across the grid circuit of the other valve; and switch means operable by said motor at its limit of operation in each direction to selectively operate one of said relay means; said switch means being operable, upon a reversal in the direction of operation of said motor, to restore the operated relay means to reconnect the grid circuit of said one valve to said signal input means and toremove such ground, and to disconnect said diode from the grid circuit of the other valve.

5. A servo-motor circuit comprising, in combination, an AC. motor having a winding controlling its direction of rotation; a pair of grid controlled electronic valves connected, in phase-detecting relation, to said winding; signal input means operable to apply a signal voltage to the grid circuits of said valves; a pair of diodes each connected to the grid circuit of one of said valves; a pair of normally closed switches each connected in series between said signal input means and one of said grid circuits; a pair of normal open switches each connected in series between one of said diodes and ground across one of said grid circuits; and limit means operable by said motor at its limit of operation'in each direction to open the normally closed switch in one grid circuit and close the normally open switch in the other circuit; said limit means being operable, upon a reversal in the direction of operation of said motor, to restore said switches to their normal condition.

6. A servo-motor circuit comprising, in combination, an A.C. motor having a winding controlling its direction of rotation; a pair of grid controlled electronic valves connected, in phase-detecting relation, to said winding; signal input means operable to apply a signal voltage to the grid circuits of said valves; a pair of diodes each con nected to the grid circuit of one of said valves; a pair of normally closed switches each connected in series between said signal input means and one of said grid circuits; a pair of first normally open switches each connected between one of said grid circuits and ground; a pair of second normally open switches each connected in series between one of said diodes and ground across one of said grid circuits; and limit means operable by said motor at its limit of operation in each direction to open the normally closed switch in one grid circuit and close the first normally open switch therein, and to close the second normally open switch in the other grid circuit; said limit means being operable, upon a reversal in the direction of operation of said motor, to restore said switches to their normal condition.

7. A servo-motor circuit comprising, in combination, an AC. motor having a winding controlling its direction of rotation; a pair of grid controlled electronic valves connected, in phase-detecting relation to said winding; signal input means operable to apply a signal voltage to the grid circuits of said valves; a pair of diodes each connected to the grid circuit of one of said valves; a pair of normally closed switches each connected in series between said signal input means and one of said grid circuits; a pair of normally open switches each connected in series between one of said diodes and ground across one of said grid circuits; a pair of relay means each operable to open the normally closed switch in one grid circuit and close the normally open switch in the other circuit; and switch means operable by said motor at its limit of operation in each direction to selectively operate one of said relay means; said switch means being operable, upon a reversal in the direction of operation of said motor, to restore the operated relay means to restore said switches to their normal condition.

8. A servo motor circuit comprising, in combination, an AC. motor having a winding controlling its direction of rotation; a pair of grid controlled electronic valves connected, in phase-detecting relation, to said winding; signal input means operable to apply a signal voltage to the grid circuits of said valves; a pair of normally closed switches each connected in series between said signal input means and one of said grid circuits; a pair of normally open switches each connected between one of said grid circuits and ground; a pair of relay means each operable to open the normally closed switch in one grid circuit and close the normally open switch therein; and switch means operable by said motor at its limit of operation in each direction to selectively activate one of said relay means; said switch means being operable, upon a reversal in the direction of operation of said motor, to deactivate the activated relay means to restore said switches to their normal condition.

9. A servo-motor circuit comprising, in combination, an AC. motor having a winding controlling its direction of rotation; a pair of grid-controlled electronic valves connected in phase-detecting relation to said winding; signal input means operable to apply a signal voltage to the grid circuits of said valves; a pair of diodes each connected to the grid circuit of one of said valves; a pair of normally closed switches each connected in series between said signal input means and one of said grid circuits; a pair of first normally open switches each connected between one of said grid circuits and ground; a pair of second normally open switches each connected in series between one of said diodes and ground across one of said grid circuits; a pair of relay means each operable to open the normally closed switch in one grid circuit and close the first normally open switch therein, and to close the second normally open switch in the other grid circuit; and switch means operable by said motor at its limit of operation in each direction to selectively activate one of said relay means; said switch means being operable, upon a reversal in the direction of operation of said motor, to deactivate the activated relay means to restore said switches to their normal condition.

References Cited in the file of this patent UNITED STATES PATENTS 2,441,269 Hartig May 11, 1948 2,534,958 Deming Dec. 19, 1950 2,594,436 Hornfeck Apr. 29, 1952 

